Investigators have developed coating compositions that are free of organic solvents, or at least incorporate a minimum amount of organic solvents, in order to avoid the environmental, toxicological, and safety problems associated with organic solvents. As a result, water-based coating compositions, including pigmented coatings, are being used in more and varied applications. In particular, water-based coating compositions, including solutions, emulsions, and dispersions, have properties that make them suitable for application to a variety of substrates, both metal and nonmetal.
However, water-based coating compositions have different application characteristics from solvent-based coating compositions. For example, many water-based coating compositions exhibit a poor flow during roller application, which results in stripes (i.e., zones of uneven film thickness) appearing during, and after, application of the coating composition to the substrate. The stripes typically remain after the curing, or air drying, of the coating composition layer. The substrate, therefore, has a coating of uneven thickness which can adversely affect the esthetics of the coated substrate and/or the barrier properties of the coating.
In order to demonstrate good flow, a coating composition applied in a roller coating application must remain in the liquid phase for a sufficient time to form a uniform layer of the coating composition on the substrate. With solvent-based coating compositions, the drying time can be adjusted, or optimized, by a judicious selection of specific solvents. In a very limited way, drying time adjustments also can be done with water-based coating compositions. However, such adjustments are not desirable and are of very limited benefit for water-based coating compositions.
It has been observed that water-based coating compositions exhibit flow problems because the coating composition dries too quickly, i.e., before the desired curing of the coating composition on the surface of the coated substrate. Accordingly, spreading the coating composition over the entire surface of the substrate to provide a coating layer of uniform thickness is hindered. For example, when a water-based coating composition is applied by a spreading blade onto a substrate, only incompletely merged strip patterns are formed, and the desired coating layer does not coat the substrate evenly.
Additional problems result because the water-based coating composition also has a tendency to dry on the application device, such that uniformity of the coating on the substrate is adversely affected. In direct, gravure, or reverse roll coating applications, the formation of a skin is evidence of drying of the coating composition on the roll surface and in the edge zones. In addition, in spray applications, drying of the coating composition produces clogged spray nozzles. Troublesome and time-consuming cleaning operations then are necessary, which adversely affect the continuous operation of the application equipment.
These problems result because the physical properties of a water-based coating composition are closely related to the properties of water. As stated above, water-based coating compositions typically have poor flow properties during roller coating application. There are several reasons for poor flow, such as high surface tension, rheological properties, and the relatively fast drying of the water-based coating compositions. An important reason for the relatively fast drying speed of water-based coating compositions is the fast evaporation rate of water, or water azeotropes, compared to solvent-based coating compositions.
It is possible to prevent or retard the undesirable, premature drying of water-based coating compositions by the addition of solvents, but this negates the primary objective of a water-based coating composition, i.e., avoiding the use of organic solvents. Accordingly, the present invention is directed to a method and apparatus that prevents premature drying of water-based coating compositions that are applied to the surface of a substrate.